1. Field of the Invention
The present invention relates to a power supply device with a DC-DC converter and a recording apparatus using the power supply device.
2. Description of the Related Art
A step-down DC-DC converter of pulse width modulation (PWM) switching type is used for power supply to a load of a drive source or an electric circuit in electronic apparatuses. The step-down DC-DC converter of PWM switching type generates a predetermined constant output voltage by performing constant value control while comparing a target voltage value for setting an output voltage with a feedback voltage value from the output voltage.
A DC-DC converter that controls an output voltage to be supplied to a load controls the output voltage by changing a target voltage value for feedback constant voltage control according to a signal from an external control unit or an external electronic apparatus. Alternatively, the DC-DC converter changes a feedback voltage value by adding current to a feedback voltage from the output voltage, thereby controlling the output voltage.
Operation of a conventional step-down DC-DC converter of PWM switching type for controlling an output voltage is described below.
FIG. 9 illustrates a basic configuration of the conventional step-down DC-DC converter 50a. The DC-DC converter 50a includes a digital to analog (D-A) converter 201 and sets a value corresponding to an output voltage value targeted by a control unit 30 to the D-A converter 201. The DC-DC converter 50a operates to maintain the output voltage value at the target voltage value so as to be stable at the target voltage. The DC-DC converter 50a changes a value to be set to the D-A converter 201, thereby changing a voltage value to be maintained.
In the step-down DC-DC converter 50a of PWM switching type illustrated in FIG. 9, an input voltage VHin supplied from a power supply unit (not-shown) is input to a switching element Q101. Then, an alternate current output converted by the switching element Q101 and a diode D101 is output via a reactor L101, so that an output voltage VH is supplied to a load 2-1.
A capacitor C001 is connected to a direct current side of the switching element Q101 and a capacitor C002 is connected to an alternate current side of the switching element Q101 via the reactor L101. The reactor L101 and the capacitor C002 constitute a smoothing circuit.
An output voltage VH detected at an output terminal of the smoothing circuit is divided by a resistor R101 and a resistor R102. The divided voltage (feedback voltage) is input to an error amplifier 202 included in a PWM control circuit (PWM control IC) 200. The PWM control circuit 200 performs feedback control to make the output voltage constant.
The circuit for performing constant voltage feedback control includes the PWM control circuit (PWM control IC) 200, which includes the error amplifier 202, a PWM comparator 203, and a triangular-wave signal generator 205, resistors R103, R104, R105, and R106, and a capacitor C003.
A discharge circuit unit H includes a switch element Q01 and a resistor R01.
One side of the switch element Q01 is connected to ground HGND and the other side of the switch element Q01 is connected to a VH output via the resistor R01. A control terminal of the switch element Q01 is connected to a control unit 30.
The switch element Q01 is turned ON or OFF in response to a DCHRG signal from the control unit 30. For example, the switch element Q01 becomes conductive when the DCHRG signal is at level H (high) and becomes nonconductive when the DCHRG signal is at level L (low).
Now, a control operation for regulating an output voltage is described below. The error amplifier 202 receives a reference voltage Vref supplied from the D-A converter 201 and a feedback voltage of the output voltage VH supplied from the resistors R101 and R102.
An output signal from the error amplifier 202 is input to the PWM comparator 203, which determines a PWM duty ratio. The PWM comparator 203 performs a comparison between an output signal from the error amplifier 202 and a triangular-wave signal output from the triangular-wave signal generator 205. The output from the PWM comparator 203, as an output signal from the PWM control circuit (PWM control IC) 200, is used to control the switching element Q101 via a metal-oxide semiconductor (MOS) drive circuit 204.
The resistors R105 and R106 and the capacitor C003, which are connected between the inverting terminal and output terminal of the error amplifier 202, constitute an exemplary phase compensation circuit.
The control unit 30, which is included in an electronic apparatus, outputs a setting signal DA_S to the D-A converter 201. The resistors R103 and R104 divide the reference voltage Vref output from an output terminal Aout of the D-A converter 201 and input the divided reference voltage Vref′ to the inverting terminal of the error amplifier 202.
The D-A converter 201 regulates a voltage of the Vref terminal based on a digital value of the setting signal DA_S and supplies the voltage to the inverting terminal of the error amplifier 202 as a voltage Vref′ divided by the resistors R103 and R104.
If the D-A converter 201 is an 8-bit D-A converter, the reference voltage Vref can be regulated in 28 stages (two to the eighth power stages), namely, in 256 stages.
A non-inverting terminal of the error amplifier 202 is connected to a voltage dividing point at which the voltage between the output voltage VH of the DC-DC converter 50a and the ground is divided by the resistors R101 and R102. The output voltage VH is expressed by equation (1).VH=Vref·(R101+R102)/R102  (1)The PWM comparator 203 performs feedback control to regulate the value of the output voltage VH to a target voltage value. The output voltage VH can be regulated in 256 stages between a maximum voltage VHmax and a minimum voltage VHmin.
An exemplary DC-DC converter in which an output voltage range of 24 V-19 V can be regulated in two to the eighth power (256) stages by using an 8-bit D-A converter is described below. In this case, a change in voltage corresponding to one bit of control data for the D-A converter is expressed by the following equation:(24 V−19 V)/28≈19.5 mV
The PWM control type DC-DC converter, in which the diode D101 is located at a low side between the switching element Q101 and the ground (HGND) as illustrated in FIG. 9, has a rather low-cost configuration. When the DC-DC converter having the above-described configuration raises an output voltage, the DC-DC converter changes the target voltage value (or changes the feedback voltage value). For example, the DC-DC converter increases an on-duty width of a MOS-FET at a high side. Accordingly, electric power is supplied from an input side to allow the output voltage to rise. A time period required for raising the output voltage to a target voltage value is determined depending on a response time of a feedback loop of the DC-DC converter.
On the other hand, in decreasing the output voltage, the output voltage VH of the DC-DC converter cannot drop immediately in response to a change of the reference voltage Vref of the D-A converter. This is because an output capacitor accumulates electric charge at a voltage generated before the output voltage drops and there is only a voltage-dividing resistor that can discharge electric charge accumulated in the output capacitor.
The voltage-dividing resistor, which determines a feedback voltage, generally employs a constant between several kΩ and several tens of kΩ so as not to degrade power conversion efficiency of the DC-DC converter. Accordingly, current flowing in the voltage-dividing resistor is several mA at the most.
Therefore, upon a light load, for example, in lowering the output voltage while a load current is 0 A, there is no path for discharging excessive electric charge having been accumulated in the output capacitor. Accordingly, it is conventionally necessary to configure a discharge circuit unit H, in which the switch element Q01 and the resistor R01 are connected in series with the output terminal as illustrated in FIG. 9, to enable discharge of the excessive electric charge accumulated in the output capacitor to the ground HGND to lower the output voltage.
Japanese Patent Application Laid-Open No. 2005-168235 discusses a configuration of the discharge circuit unit H, which discharges electric charge of a capacitor at the output terminal when lowering the output voltage.
In Japanese Patent Application Laid-Open No. 2005-168235, a signal for driving the discharge circuit unit H has a constant pulse width independent from a setting value of the output voltage so as to step down the output voltage to a target voltage value within a defined time period in throughout a voltage range required as the output voltage of the DC-DC converter.
In the above-described DC-DC converter, which regulates the output voltage, a control unit preliminarily determines a pre-set voltage. For example, the DC-DC converter has a function of regulating the output voltage to perform energy correction with respect to variation of parts at the load side and environmental variation.
The above-described DC-DC converter, which supplies a power supply voltage to an electronic apparatus, is required to change the output voltage in a short time period in response to a command from the control unit while the DC-DC converter is outputting a voltage of a certain value.
Operation of the DC-DC converter is described with reference to waveforms illustrated in FIG. 10 with regard to a case where the control unit outputs a setting signal to change the output voltage to an output voltage V1 (Vo>V1) when the DC-DC converter is outputting a certain output voltage Vo.
Prior to receiving the setting signal (DA_S) for changing the output voltage, if a load current of the DC-DC converter is 0 A, the DC-DC converter can maintain the output voltage level if the DC-DC converter is supplied with an amount of electric power corresponding to that having been lost in the DC-DC converter. Therefore, the switching element Q101 is in a state of OFF operation, namely, duty 0% operation, almost throughout the switching cycle.
If the DC-DC converter receives, from the control unit 30, the setting signal (DA_S) for changing the output voltage from Vo to V1 (Vo>V1) between time t0 and time t1 illustrated in FIG. 10, an output value from the D-A converter 201 becomes smaller (not shown). Accordingly, the control unit 30 changes the target setting voltage of the DC-DC converter.
After the control unit 30 sets the target setting voltage of the DC-DC converter, the control unit 30 transmits a DCHRG signal (discharge command) of a preliminary set constant pulse width for a time interval between time t2 and time t4.
When the switch element Q01 receives the DCHRG signal at level H from the control unit 30 at time t2, the switch element Q01 becomes conductive. Since the load current is not extracted from the output between time t1 and time t4, the VH voltage remains at Vo between time t1 and time t2 before receiving the DCHRG signal. When the switch element Q01 becomes conductive at time t2, discharge current flows to the ground HGND via the resistor R01 to allow a potential of the capacitor C002 (VH) to drop from Vo to the target voltage V1, so that the output voltage reaches the target voltage V1 at time t3.
The output voltage in a time interval between time t2 and time t3 is expressed by equation (2):V1=Vo·exp(−t/(C002·R01))  (2)The current flowing in the resistor R01 becomes I R01=V1/R01 according to the voltage V1.
Since the control unit 30 outputs the pulse width of the DCHRG signal between time t2 and time t4, the output voltage immediately drops to the target voltage V1 before a time interval between time t3 and time t4. At that time, the DC/DC converter is still performing constant voltage control at the target voltage V1, the output voltage V1 is continuously applied to the resistor R01.
The switch element Q01 is turned OFF when the DCHRG signal reaches level L at time t4, thus resulting in terminating a series of VH modulation control. In the above description, it is assumed that an on-resistance of the switch element Q01 is 0Ω.
The output voltage change range (output voltage regulating range) of the DC-DC converter illustrated in FIG. 9 is between the maximum value VHmax and the minimum value VHmin. In stepping down the output voltage VH from a certain initial voltage Vo to the target voltage V1, the pulse width required to step down the output voltage VH to the target voltage V1 can be expressed by equation (3). Here, a capacitance of the output capacitor C002 is C002 [μF], a discharge resistance value is R01 [Ω], and the on-resistance of the switch element Q01 of the discharge circuit unit H is ignored.t=−C002·R01·LN(V1/Vo)  (3)
For example, 220 μF of the capacitance and 100Ω of the discharge resistance value R01 are applied to the above formula, a pulse width of 0.936 ms is required, for example, when the output voltage is lowered by 1.0 V, namely, from 24 V to 23 V. Further, a pulse width of 1.128 ms is required when the output voltage is lowered by 1.0 V, namely, from 20 V to 19 V.
In view of equation (3), it is understood that, as a potential difference between the initial voltage Vo and the target voltage V1 becomes larger, a time period required for stepping down the output voltage VH becomes longer. It is also understood that, as the capacitance of the output capacitor C002 becomes larger, the pulse width for driving the discharge circuit unit H becomes longer.
As described above, a conduction time of the discharge circuit unit H for stepping down a constant voltage varies with an initial setting and a target voltage if the output capacitor C002 is defined.
For example, a DC-DC converter that is capable of changing an output voltage between the maximum value VHmax of 24 V and the minimum value VHmin of 19 V requires the longest discharge time to lower the VH voltage from 24 V to 19 V.
Here, if this condition is applied to the above circuit constant, the DC-DC converter requires a discharge time of 5.132 ms to lower the voltage by 5 V, namely, from 24 V to 19 V. Thus, the DC-DC converter drives the discharge circuit unit H at a constant pulse width of 5.132 ms to change (regulate) the VH voltage output range to the target voltage.
In other words, the DC-DC converter requires a pulse width of 5.132 ms to lower the voltage to the target voltage value within the time period defined by the constant pulse width in the changeable range of the DC-DC converter.
A DC-DC converter discussed in Japanese Patent Application Laid-Open No. 2005-168235 is described below. The DC-DC converter continuously drives the discharge circuit unit H at a constant pulse width to lower the output voltage to a target voltage value within a predetermined time period in the output voltage range of the DC-DC converter.
If a DC-DC converter in which the output voltage range between 24 V and 19 V can be regulated by an 8-bit D-A converter into 28 (256) stages is exemplified as the DC-DC converter configured to regulate the output voltage, a voltage change for one bit of the D-A converter is expressed by the following formula:(24 V−19 V)/28≈19.5 mV
In other words, the DC-DC converter drives the discharge circuit unit H at the constant pulse width of 5.132 ms to step down the output voltage both in the case of stepping down the output voltage of the DC-DC converter from 24 V to 19 V and in the case of stepping down the output voltage of the DC-DC converter from 24 V by 19.5 mV.
Here, a maximum time period t for stepping down the output voltage by driving the discharge circuit unit H with a constant pulse width throughout the whole output voltage range can be expressed by equation (4).t=−C002·R01·LN(VHmin/VHmax)  (4)
Consequently, the pulse width of the DCHRG signal for stepping down the set voltage value is determined according to a conduction time that satisfies a variation width (ΔVHmax) based on the maximum value (VHmax) and the minimum value (VHmin) of the voltage range. Namely, in this example, it is a conduction time of 5.132 ms that satisfies a change of the voltage from 24 V to 19 V.
Electric power to be applied to the discharge resistor is described below. As illustrated in FIG. 9, the output voltage of the DC-DC converter, configured to regulate the output voltage, has an output voltage width between the maximum value VHmax and the minimum value VHmin. Therefore, the voltage value applied to the discharge resistor upon conduction of the switch element Q01 is also a voltage value between the maximum value VHmax and the minimum value VHmin. However, the applied voltage is not always constant because of the DC-DC converter configured to modulate the output voltage. For example, in the case of a DC-DC converter capable of changing the output voltage between 24 V and 19 V, the voltage between 24 V and 19 V may be applied to the discharge resistor.
Here, a case where the discharge resistance is 100Ω is described. When the output voltage is 24 V, electric power continuously applied to the discharge resistor is 5.76 W. When the output voltage is 19 V, electric power continuously applied to the discharge resistor is 3.61 W. The continuously applied electric power in the case where the output voltage is 24 V becomes about 1.6 times as a case where the output voltage is 19 V.
In the discharge circuit unit H, including the switch element and the resistor connected in series, the output voltage VH is continuously applied to the discharge resistor while the switch element of the discharge circuit unit H is conductive even if the output voltage drops and reaches the target value during a step-down process of the output voltage. Accordingly, a constant discharge current continues to flow in the discharge resistor, namely, a constant electric power is continuously applied to the discharge resistor.
In a typical characteristic of the resistor, a limited electric power at one-pulse is defined by pulse limiting electric power curves as illustrated in FIG. 11. FIG. 11 illustrates a mere example of pulse limiting electric power curves of a lead resistor, namely, pulse limiting electric power curves of five types of rated power between 0.17 W and 2 W and resistor sizes. The characteristic is defined for each respective resistor regardless of types of resistors (for example, metal film, carbon, oxidative metal, and fusing resistor) or manufacturers thereof.
In the typical resistor, as the electric power application time becomes longer, the limiting electric power decreases more. Also, as the rated power becomes higher, the resistor size becomes larger and the limiting value of the pulse limiting electric power curve becomes high. However, if the resistor size becomes larger, there is required a larger installation space, thus resulting in higher cost.
If the output capacitance C002 and the output voltage range (VHmax, VHmin) are determined according to formula (4), the time period t required for stepping down the output voltage is determined according to the resistance value of the discharge resistor.
The maximum electric power is applied to the resistor at a time when the VH voltage for one bit of the D-A converter (19.5 mV) is stepped down from the maximum voltage VHmax.
FIG. 12 illustrates a calculating result of a discharge time required for stepping down the voltage in the whole voltage range and electric power applied to the discharge circuit unit H for respective different constants of the discharge resistor R01.
A required discharge time is represented by a pulse width value in FIG. 12 calculated with the proviso that the maximum value VHmax is 24 V, the minimum value VHmin is 19 V, and the capacitor C002 is 220 μF in formula (1), for respective resistance values of the resistor R01 between 10Ω and 220Ω.
For the ease of calculation, the resistor applied electric power is a value of electric power applied to the resistor when the discharge circuit unit H is made conductive while the output voltage remains at 24 V. That is, the applied electric power is calculated by 24 V×24 V/R01 to obtain the maximum electric power to be applied to the resistor.
FIG. 13 is a graph obtained by plotting the result illustrated in FIG. 12 into pulse limiting electric power curves. As apparent from FIGS. 12 and 13, as the resistance value becomes larger, the applied electric power becomes smaller, thus resulting in enabling the use of a resistor having a smaller rated power. However, it is seen that the conduction time of the discharge circuit unit H, namely, a pulse width required for driving the discharge circuit unit H for stepping down the output voltage, is elongated.
Further, as the resistance value becomes smaller, the output voltage drops in a short time period. However, the electric power to be applied to the resistor becomes larger. Therefore, the use of such a resistor having both a large rated power and a large resistor size is required.
As understood from the above description, it is required to make the resistance value of the electric discharge resistor smaller to drive the discharge circuit always at a constant pulse to step down the output voltage to the target output voltage value within a predetermined time period in the output voltage range of the DC-DC converter.
However, in using the resistor within the pulse limiting electric power curve, a resistor having a large rated power and a large resistor size is required. Also, in using a resistor requiring a small installation space, the electric power to be applied to the resistor is required to be limited within the pulse limiting electric power curve, such that the time period for stepping down the output voltage to the target value tends to take time in a conventional configuration.
Considering the above-described point, in order to avoid increase of the sizes of the rated power and, thus, a power source unit, it is required to shorten the discharge time by connecting small-sized rated power resistors in series to reduce the electric power to be applied to the resistors. Thus, a problem of increased cost arises.